Endothelin binding sites in porcine aortic and rat lung membranes

Immunoreactive endothelin-1 and its receptors in human adrenal tissues

To elucidate the pathophysiologic significance of endothelin-1 (ET-1) in adrenal and the mechanism for reduced responsiveness to exogenous ET-1 in aldosterone-producing adenoma (APA), we have investigated ET-1 receptors by radioligand binding assay (RBA) in human normal adrenal (NA), APA, idiopathic hyperaldosteronism (IHA), and pheochromocytoma (PHEO), immunoreactive (ir-) ET-1 content in NA, APA and PHEO by radioimmunoassay (RIA), and immunohistochemical staining of ET-1 with the peroxidase-anti-peroxidase (PAP) method in NA, APA, and PHEO. A single class of high-affinity binding sites for ET-1 was found in human NA and tumor tissues. Dissociation constant (Kd) values of ET-1 receptors were similar in NA, APA, and IHA, but maximal binding capacity (Bmax) of ET-1 receptors was lower in APA than in NA and IHA. Both Kd and Bmax in PHEO were higher than those in NA, APA, and IHA. Ir-ET-1 content in tumors of APA and PHEO were higher than in NA. Immunohistochemical staining was more intense in the tumor cells of APA and PHEO than in NA. These results suggest that the reduced response to exogenous ET-1 in APA could be related to downregulation of ET-1 receptors in the tumor. Increased ET-1 content and receptors may lead to hypersecretion of catecholamine in PHEO. ET-1 produced in normal and tumor adrenal tissues may regulate aldosterone and catecholamine secretion from adrenals in a paracrine/autocrine fashion.

Identification of endothelin receptor subtypes in rat ciliary body using subtype-selective ligands

The endothelins are important vasoactive ocular peptides and there is some evidence that they may modulate intraocular pressure. We investigated the existence and localization of endothelin receptor subtypes using subtype selective ligands in rat ciliary body. Scatchard transformation of saturation binding experiments revealed that the KD and Bmax for [125I]ET-1 and [125I]ET-3 to membranes from ciliary body were 41.7+/-9 pM and 236+/-20 fmol mg-1 protein and 37. 8+/-0.4 pM and 160+/-2.0 fmol mg-1 protein, respectively. Competitive experiments in the presence of cyclic pentapeptide BQ123 (selective for ETA receptors) and BQ3020 (selective for ETB receptors), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. Cross-linking of [125I]ET-1 and [125I]ET-3 to ciliary body membranes resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa, suggesting that ETA and ETB receptors have similar molecular mass. The 34 Kda band is a proteolytic degradation product of the 52 Kda band. Autoradiographic results show that specific [125I]ET-1 binding sites, displaced by BQ123 and BQ3020, are localized to the ciliary epithelium, supporting the idea that ETA and ETB subtype receptors exist in this tissue.

Inhibitors of endothelin

With the advent of the first generation of both selective and nonselective endothelin antagonists being a relatively recent event, the manifold therapeutic potentials of these compounds are only now being explored clinically. Undoubtedly, numerous clinical utilities for these compounds will soon be realized.

Nitric oxide and endothelin in pathophysiological settings

The role of the endothelium is now known to encompass the generation of many potent cytokines which impact endothelial cells, adjacent tissue such as smooth muscle cells, and distant sites in an autocrine, paracrine, and endocrine manner, respectively. This review addresses two of these cytokines, nitric oxide and endothelin, and describes how each effects the functions of endothelial cells, including regulation of platelet aggregation and coagulation, regulation of vasomotor tone, modulation of inflammation, and the regulation of cellular proliferation. The emphasis is on the increasingly recognized importance of the autocrine and paracrine mechanisms by which nitric oxide and endothelin act. In particular, autoinduction of endothelin is proposed as a central mechanism underlying endothelin's renowned effects. Additionally, specific nitric oxide/endothelin interactions are discussed by which each cytokine modulates the production and actions of the other. The net effect observed in a variety of physiological and pathophysiological settings, therefore, reflects a balance of these opposing functions.

Characterization of endothelinA receptors in cerebral and peripheral arteries of the rat

We have characterized and quantified endothelin receptors in rat brain (anterior cerebral) and peripheral (aorta, carotid, and caudal) arteries, with the use of [125I]endothelin and quantitative autoradiography. Endothelin binding was saturable, of high affinity, and totally displaced by the selective endothelin ETA antagonist BQ 123. A single class of ETA receptors is located in the medial layer of peripheral and cerebral arteries, and its quantification by autoradiography allows study of their regulation and function.

Plasma endothelin levels in congenital diaphragmatic hernia

Persistent pulmonary hypertension (PPH) together with pulmonary hypoplasia accounts for high mortality among patients with congenital diaphragmatic hernia (CDH). Endothelin (ET) is a recently described endothelium-derived vasoconstrictor peptide with a potent and sustained effect. The authors examined plasma ET levels in eight patients with CDH who had respiratory distress at birth and in 15 age-matched controls. The plasma ET levels were measured using a highly sensitive and specific enzyme immunometeric assay (EIA) preoperatively, intraoperatively, and 24 and 48 hours postoperatively. The mean preoperative plasma ET level (53.9 +/- 21.1 pg/mL) of the patients was significantly (P < .01) higher than that of controls (5.9 +/- 4.5 pg/mL). Two patients who died before operation had plasma ET levels of 60 pg/mL and 82 pg/mL, respectively. Three patients who had PPH postoperatively continued to have high ET levels, and at 48 hours, the mean plasma ET level in these patients was 33 pg/mL. In contrast, in the three patients who survived after repair of CDH, the mean plasma ET level at 48 hours was 1.3 pg/mL. The authors further examined the lungs of two patients who died before operation for the expression of ET, using immunocytochemistry. Pulmonary endothelial cells had strong ET staining compared with control tissues. These findings suggest that ET may be a pathophysiological mediator of pulmonary hypertension in patients with CDH.

Termination of endothelin signaling: role of nitric oxide

Cellular mechanisms responsible for the termination of ET-1 signal are poorly understood. In order to examine the hypothesis that nitric oxide serves as a physiological brake of ET-1 signaling, Chinese hamster ovary (CHO) cells stably transfected with the ETA receptor cDNA (CHO-ET) were studied. CHO-ET responded to ET-1 with robust [Ca2+]i transients and developed a long-lasting homologous desensitization. Donors of nitric oxide (NO), 3-morpholino-sydnonimine HCl (SIN-1), or sodium nitroprusside (SNP) reduced the amplitude of these responses, accelerated the rate of [Ca2+]i recovery, and counteracted the development of homologous desensitization by a cyclic GMP-independent mechanism, suggesting an alternative mode for NO modulation of ET-1 responses. Stimulation of CHO-ET cells with mastoparan, a wasp venom acting directly on G proteins (bypassing receptor activation), was inhibited by NO, revealing a postreceptoral target for NO-induced modulation of [Ca2+]i mobilization. Using a lys9-biotinylated ET-1 (ET-1 [BtK9]), binding sites were "mapped" in CHO-ET cells. Receptor-ligand complexes did not exhibit spontaneous dissociation during 60-min observations. Quantitative fluorescence microscopy revealed that SNP or SIN-1 caused a rapid, concentration-dependent, and reversible dissociation of biotinylated ET-1 from ETA receptor (EC50 = 75 microM and 6 microM, respectively), an effect that was not mimicked by 8-bromo-cyclic GMP. "Sandwich" co-culture of endothelial cells with CHO-ET showed that activation of NO production by endothelial cells similarly resulted in dissociation of ET-1 [BtK9] from ETA receptors. We hypothesize that NO plays a role in physiological termination of ET-1 signalling by dual mechanisms: (1) displacement of bound ET-1 from its receptor, thus preventing homologous desensitization, and (2) interference with the postreceptoral pathway for [Ca2+]i mobilization, hence inhibiting end-responses to ET-1.

Localization and characterization of binding sites for endothelin in Harder's gland in rabbits

The characterization and localization of binding sites for endothelin-1 (ET-1) labeled with iodine 125I were investigated in homogenized tissues and sections of Harder's glands of normal rabbits. The membrane of Harder's glands was harvested and incubated with 125I-ET-1 (0.25-1 nmol/L) in 20 +/- 4 mg of protein per 0.25 mL at 37 degrees C for 90 min in the presence of protease inhibitors. Specific labeling was assessed by coincubating unlabeled ET-1, ET-2, ET-3 and other unrelated cytokines. The tissue labeled with 125I-ET-1 was collected by filtration and counted in a gamma counter. For an in vitro autoradiography study, 15 microns cryostat sections were incubated with 125I-ET-1 (0.1 nmol/L). They were fixed, dipped in liquid emulsion and kept for 6 days before development. Membrane counting showed that the binding of 125I-ET-1 to Harder's gland was saturable. Scatchard data analysis revealed one class of binding with a dissociation constant (Kd) of 0.33 nmol/L and a maximal density of binding (Bmax) of 794 attomole/mg of protein. The binding was inhibited most by ET-1, followed by ET-2 and then ET-3 but not by unrelated peptides. Emulsion-dipped slides with sections showed specific high-density labeling mainly over structures identified from serial sections stained by hematoxylin-eosin as the walls of capillaries, arterioles, arteries, and veins of the glands. Less dense binding was found in both white and pink lobes of the gland. No binding was found in fat and connective tissues. The distribution of endothelin action sites in the glandular blood vessels and Harder's gland suggests that the peptide may have a role in the regulation of blood circulation and glandular secretion in the normal rabbit.

Relative contributions of direct and indirect mechanisms mediating endothelin-induced contraction of guinea-pig trachea

1. The present study was undertaken to determine the mechanism of action of endothelin-1 (ET-1)-induced contraction of the guinea-pig isolated trachea. 2. ET-1 (1 nM-0.3 microM) produces a concentration-dependent contraction of guinea-pig trachea with an EC50 of approximately 25 nM. The combination of the peptidoleukotriene receptor antagonist, SK&F 104353 (10 microM) and the H1-histamine receptor antagonist, mepyramine (10 microM), which abolishes antigen-induced contraction in guinea-pig trachea, was without effect on ET-1 concentration-response curves. Furthermore, the platelet-activating factor (PAF) receptor antagonist, WEB 2086, (1 or 10 microM) did not inhibit ET-induced contraction. 3. ET-1 (0.3 microM) did not stimulate histamine or immunoreactive peptidoleukotriene release from guinea-pig isolated trachea. 4. The release of various prostanoids from guinea-pig trachea was increased significantly by ET-1 (0.3 microM); the profile of release was prostaglandin D2 (PGD2) = PGE2 = 6-keto PGF1 alpha (PGI2 metabolite) > thromboxane B2 = PGF2 alpha >> 9 alpha, 11 beta PGF2 (PGD2 metabolite). ET-1-induced release of prostaglandins, which was about 30% of that elicited by antigen in sensitized tissues, was not affected by epithelium removal and was observed in tissues from which the smooth muscle had been removed. Previous studies in our laboratory indicated that indomethacin potentiated contraction produced by high concentrations of ET-1, whereas a thromboxane receptor antagonist was without appreciable effect on ET-1 concentration-response curves. 5. Pretreatment of tissues for 1 h with capsaicin (10 microM), which depletes different sensory neurones, produced a small, but significant, inhibitory effect on ET-1 concentration-response curves in the presence but not the absence of the epithelium. The combination of the NK1 tachykinin receptor antagonist,CP-96,345 (0.1 microM), and the NK2 tachykinin receptor antagonist, SR 48968 (0.1 microM), was without effect on ET-l concentration-response curves but substantially antagonized capsaicin-induced contraction.6. The present data suggest that in guinea-pig isolated trachea, ET- 1 produces contraction predominantly via a direct mechanism: there is no significant contribution of the release of histamine,leukotrienes, PAF, or tachykinins (acting on NK1 or NK2 receptors). Although ET-1 evokes the release of an array of prostanoids from the trachea they do not appear to have a major influence on the contractile response.

Identification and characterization of endothelin receptor subtype B in rat retina

The presence of immunoreactive (IR) endothelin (ET)-1 and ET-1 receptors in rat retina has been studied by radioimmunoassay and receptor assay, respectively. The specific binding of 125I-ET-1 to rat retinal particulate preparations was saturable. Apparent equilibrium conditions were established within 120-140 min. Scatchard analysis of binding data indicated a single class of high-affinity binding sites with a KD of 35 +/- 11 pM and a Bmax of 168 +/- 60 fmol/mg of protein. 125I-ET-1 binding to retinal particulate preparations was not inhibited by 1 microM concentrations of somatostatin, atrial natriuretic factor, brain natriuretic peptide, thyroid-stimulating hormone, growth hormone, or insulin. The three endothelin isoforms, ET-1, -2, and -3, had similar affinity for the receptor. Cross-linking of 125I-ET-1 to retinal particulate preparations with disuccinimidyl suberate resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa. We have established a highly sensitive and specific radioimmunoassay for ET-1. The concentration of IR-ET-1 in rat retina was 35 +/- 10 fmol/g wet weight. The demonstration of specific high-affinity ETB receptors and the presence of IR-ET-1 suggest that the peptide may act as a neurotransmitter or neuromodulator in the retina.

Transient forebrain ischemia alters acutely endothelin receptor density and immunoreactivity in gerbil brain

Pharmacologic studies suggest that endothelin (ET) plays an important role in the pathophysiology of hemorrhagic and ischemic stroke. In the gerbil, transient forebrain ischemia (10 min) resulted in profound motor deficits and a 15% reduction in ET receptor density in the hippocampus at 60 min post-reperfusion. A significant 2-fold increase in forebrain immunoreactive ET accompanied the maximum post-ischemic decrease in ET receptor density. These results suggest that the synthesis and availability of ET are increased acutely in the forebrain following transient cerebral ischemia.

Differential effects of endothelin-1 on the vasorelaxant properties of benzopyran and non-benzopyran potassium channel openers

1. The effects of endothelin-1 (ET-1) on the vasorelaxant properties of structurally different potassium channel openers (PCOs), BRL-38227, Ro 31-6930, SDZ PCO 400, EMD-52692, RP-49356 and pinacidil, were studied. 2. All PCOs evoked concentration-related relaxations of ET-1 (10 nM) or KCl (20 mM) contracted rat isolated aortic rings denuded of endothelium. BRL-38227, EMD 52692, SDZ PCO 400 and Ro 31-6930 were 11-42 times less potent in relaxing contractions to ET-1 than KCl. In contrast, this differential potency was not observed with RP-49356 or pinacidil. 3. BRL-38227 (0.06-3.0 microM), RP-49356 (0.3-3.0 microM) and pinacidil (0.3-3.0 microM) displaced KCl concentration-response curves to the right of controls, without modifying the maximum response. A subcontractile concentration of ET-1 (0.1 nM) prevented the inhibitory effects of low concentrations of BRL-38227 (0.06-0.1 microM) on KCl responses, but failed to modify those to RP-49356, pinacidil or high concentrations of BRL-38227 (0.3-3.0 microM). The inhibitory effects of BRL-38227 (0.1 microM) were also not changed by ET-3 (1.0 nM) or angiotensin II (0.1 nM). 4. In anaesthetized spontaneously hypertensive rats (SHR), cumulative bolus intravenous administrations of BRL-38227 (1-1000 micrograms kg-1, i.v.), Ro 31-6930 (1-1000 micrograms kg-1, i.v.), RP-49356 (10-1000 micrograms kg-1, i.v.) or nitrendipine (0.1-30 micrograms kg-1, i.v.) produced dose-dependent falls in diastolic blood pressure (DBP).ET-1 (i.v.) evoked a transient fall in DBP (1 pg kg- = 58 + 1 mmHg) which returnedto pre-administration levels within 4 min.5. Pretreatment of anaesthetized SHR with ET-l (1 pg kg-', i.v.) significantly increased the ED,5 (dose to evoke a 15% fall in DBP) values for BRL-38227 and Ro 31-6930. However, ET-l failed to modify the ED,5 values for RP-49356 or nitrendipine. The ED50 values for all of the vasodilators studied were not modified by ET-1.6. Infusion of BRL-38227 (2 pgkg-'min-', i.v.) or RP-49356 (4 pgkg-'min', i.v.) to anaesthetized SHR evoked dose-related falls in DBP, with a corresponding increase in descending aortic blood flow (DABF) and a decrease in total lower body vascular resistance (TLBVR). Pretreatment with ET-1 (1 ptg kg-', i.v.) significantly attenuated the decreases in DBP and TLBVR observed with low doses of BRL-38227, but not RP-49356 or high doses of BRL-38227. In contrast, ET-3 (3 pig kg-, i.v.) failed to modify the effects of BRL-38227 on DBP or TLBVR.7. In conscious SHR, the fall in DBP to BRL-38227 (30 pgkg-', p.o.) was significantly reduced following ET-1 (1 pig kg-', i.a.) treatment. ET-1 (1 pg kg-', i.a.) pretreatment, however, failed to modify the decrease in DBP induced by an equieffective oral dose of RP-49356 (1001pgkg-1).8. In conclusion, ET-1 selectively attenuated the vasorelaxant effects of the potassium channel opener,BRL-38227 and other substituted benzopyrans. The results are compatible with the hypothesis that benzopyran PCOs and ET-1 have affinity for a site that does not recognise RP-49356 or pinacidil. Thus,ET-l can differentiate between structurally unrelated potassium channel openers. The cardiovascular effects of some, but not all, PCOs might be radically modified in the clinical setting by elevated endogenous levels of ET-1 associated with certain diseased states.

A reversible radioligand specific for the ETB receptor: [125I]Tyr13-Suc-[Glu9,Ala11,15]-endothelin-1(8- 21), [125I]IRL 1620

Suc-[Glu9,Ala11,15]-endothelin(ET)-1(8-21), IRL 1620, is a linear ET-analog specific for the ET-isopeptide-nonselective ETB receptor. The radio-iodinated analog, [125I]IRL 1620, showed a single class of saturable binding to the ETB receptors in porcine lung membranes with a Kd of 18 pM and a Bmax of 930 fmol/mg protein, which are almost comparable to the values obtained with [125I]ET-3 (6 pM and 900 fmol/mg protein). In competitive binding assays with [125I]IRL 1620, unlabeled ET-1, ET-3, IRL 1620 and [monoiodo-Tyr13]-IRL 1620 showed almost identical displacement curves with Ki of 8 to 16 pM. However, [125I]IRL 1620 was dissociated from the binding sites by addition of an excess amount (100 nM) of any of these unlabeled peptides, each with the same t1/2 of 100 min. This was in marked contrast to [125I]ET-3 which was hardly dissociated from the binding sites.

Intracellular distribution of endothelin-1 receptors in rat liver cells

We studied the binding of (125I)-endothelin-1 as well as that of the vasopressin analogue (125I)-[8-phenylpropionyl]-LVP to purified plasma membranes, Golgi cisternae and cell nuclei from rat liver. Cell organelles were isolated by differential centrifugation and discontinuous sucrose gradients. Endothelin-1 exhibited specific binding to plasma membranes, Golgi cisternae and nuclei, while the binding of (125I)-[8-phenylpropionyl]-LVP was restricted to the plasma membranes. The number of receptors (Bmax) and the binding constants (Kd) were determined by Scatchard analysis of competition binding studies. In all cases only one class of Et-1 binding sites could be detected. The presence of Et-1 receptors on the Golgi complex either indicates that the receptor is glycosylated within the cisternae or alternatively, there exists a recycling pathway. The unexpected finding of Et-1 receptors on highly purified nuclei suggests that this peptide may exert part of its biological functions intracellularly via the nucleus.

Production of endothelin-1 and big-endothelin-1 by pleural mesothelial cells

Immunoreactive endothelin-1 (ET-1) and big-endothelin-1 (big-ET-1) were detected in conditioned medium of cultured rat pleural mesothelial cells by using sensitive sandwich-type enzyme immunoassay. The amount of both ET-1 and big-ET-1 increased time-dependently. In both instances, maximal amount was detectable in conditioned medium obtained after 72 h in culture (ET-1: 117.1 +/- 30.1 pg/10(6) cells; big-ET-1: 2.4 +/- 2.1 pg/10(6) cells). Fetal calf serum markedly stimulated the production of both ET-1 and big-ET-1.

Localisation of endothelin like immunoreactivity in adult and developing human gut

The distribution of immunoreactivity for the potent vasoconstrictor endothelin-1 was studied in adult and developing human gut using antisera to endothelin-1 (1-21) and the C terminus of big endothelin-1. The coexistence of these peptides with other neuropeptides was investigated using comparative immunocytochemistry. Endothelin-1 like immunoreactivity was detected in extracts of adult (range 20-60 fmol/g wet weight) and fetal (33 fmol/g) gastrointestinal tract and was shown by chromatography to be the predominant isoform of endothelin present in both. It was localised by immunocytochemistry to ganglion cells in the submucous and myenteric plexuses and to scattered nerves, whereas big endothelin-1 like immunoreactivity was found in the submucous plexus only. Colocalisation studies showed immunoreactivity for both endothelin-1 and vasoactive intestinal peptide in the same ganglion cells of the submucous plexus. Although endothelin-1 immunoreactivity was not detected by immunocytochemistry in the fetal human gut until the 32nd week of gestation, big endothelin-1 was found as early as 11 weeks in the developing neural structures and epithelial cells. The latter were shown to be endocrine cells by their immunoreactivity for chromogranin. Our results indicate that endothelin is a neuropeptide found in adult human gut which shows transient expression in endocrine cells during development.

Kinetic analyses demonstrate that the equilibrium assumption does not apply to [125I]endothelin-1 binding data

The kinetics of [125I]Endothelin-1 ([125I]ET-1) binding were studied using membranes from rat heart, rat lung, rat brain, and porcine vascular smooth muscle at 37 degrees C in 0.05M Tris-HCl buffer (pH = 7.4). The dissociation half-life (t1/2, diss.) for bound [125I]ET-1 was in excess of 30 hours for each tissue studied. Equilibrium-time requirements for proper Scatchard analysis of [125I]ET-1 were also far in excess of 30 hours for each tissue. These data suggest that determination of dissociation constants, Kd, and receptor concentrations, Bmax, by conventional Scatchard analysis is not feasible with [125I]ET-1. Kinetic analyses may provide a more accurate means for determining [125I-ET-1] binding characteristics including Kd and Bmax.

The irreversibility of endothelin action is a property of a late intracellular signalling event

The irreversibility of the contractile action of endothelin-1 (Et) and of its binding to its receptors are usually believed to be linked in a direct manner. Rat aortic strips were exposed to 25 nM Et for short periods of time that were sufficient to irreversibly saturate membrane receptor sites and then washed of unbound Et. Under these conditions, fast and transient contractile responses were observed. They were unlike the slow and irreversible contractions observed in the continued presence of the peptide. They were as fast as KCl, angiotensin II and vasopressin contractions. It is concluded that the irreversibility of Et contractions and of its interaction with its receptors can be uncoupled. The data also suggests that recycling of endocytosed Et receptors contributes to the sustained contractile action of the peptide.

Importance of the phenotypic state of vascular smooth muscle cells on the binding and the mitogenic activity of endothelin

Smooth muscle cells of the rabbit aorta, when grown in vitro, express distinguishable forms of phenotypes (contractile and synthetic). On contractile cells, ET-1 specifically bound to a single class of high affinity (KD = 128 pM) and high capacity (Bmax = 66,000 sites/cell) binding sites. But, whereas affinity of [125I]-ET-1 was not significantly affected by phenotypic modulation, synthetic cells displayed a 10-fold lower [125I]-ET-1 binding capacity than contractile smooth muscle cells. Similarly, the mitogenic effect of ET-1 on smooth muscle cells was considerably lower for synthetic than for contractile cells. The ET-1 receptor on primary cells was recognized by sarafotoxin S6b and the different ET-related peptides with an order of potency [ET-1 greater than S6b greater than ET-3 greater than Big ET-1 much greater than ET(16-21)] identical to that inducing smooth muscle cell growth. Therefore, these data indicate that the binding and the mitogenic effects of ET-1 on smooth muscle cells might be of different magnitudes depending on the phenotypic state of these cells.

Intracellular ramification of endothelin signal

Effects of porcine 1-21 endothelin (ET-1) on [Ca2+]i, [Na+]i, and [Cl-]i and on membrane potential were studied in individual mesangial (MC) and vascular smooth muscle (VSMC) cells using microspectrofluorimetry of fura-2, SBFI, SPQ, and bis-oxonol, respectively. ET-1 increased [Ca2+]i by fivefold, showing an immediate and a sustained phase of response. Ca(2+)-free medium and nifedipine pretreatment significantly curtailed the sustained phase of response to ET-1. These findings were confirmed in studies of vascular ring preparations, demonstrating that Ca2+ influx may account for at least 50% of contraction. ET-1 caused immediate and sustained depolarization of MC and VSMC. This could not be attributed to Na+ influx, since fluorescence of SBFI was not affected by ET-1 and Na(+)-free medium did not abolish the ET-1-induced membrane depolarization. Studies of SPQ fluorescence changes induced by ET-1 revealed an increase in fluorescence intensity consistent with the decrease in [Cl-]i. A Cl- channel blocker, IAA-94, abolished changes in SPQ fluorescence and curtailed sustained phases of membrane depolarization and [Ca2+]i elevation in response to ET-1, but did not affect KCl-induced [Ca2+]i transients. IAA-94 also attenuated the ET-1-induced contraction of aortic rings. Microinjection of either calcium gluconate or inositol 1,4,5-trisphosphate (IP3) in SPQ-loaded cells resulted in an increase in fluorescence mimicking the effect of ET-1. These changes were blunted by pretreatment of cells with BAPTA and incubation in Ca(2+)-free medium. When IP3 was microinjected into fura-2-loaded MC, this resulted in immediate and sustained elevation of [Ca2+]i. In conclusion, generation of IP3 results in mobilization of intracellular Ca2+ stores and activation of Cl- channels. Ensuing Cl- efflux causes membrane depolarization and, in turn, activation of voltage-dependent Ca2+ channels, resulting in sustained elevation of [Ca2+]i which is indispensable for the full-scale contraction produced by ET-1.

The bovine endothelin receptor has an apparent molecular weight of 43,000

In crosslinking experiments, [125I]endothelin-1 was treated with N-hydroxysuccinimidyl-4-azidobenzoate, purified by HPLC, allowed to bind to bovine aortic membranes and then photoactivated. Autoradiography of sodium dodecyl sulfate polyacrylamide gel electrophoretograms of the products of this reaction showed that a component of apparent Mr = 42,000 was specifically labelled by endothelin-1 under reducing conditions. Under nonreducing conditions, a small amount of 125I-labelled endothelin-1 specifically labelled a component of apparent Mr = 45,900 in the absence of crosslinking agent. Non-radiolabelled endothelin analogues with a wide range of binding affinities inhibited specific labelling of the Mr = 42,000 and 45,900 components in parallel over the concentration ranges which inhibited binding of radiolabelled endothelin. Specific labelling of these components was also observed in parallel in membranes from bovine heart and kidney. The components labelled in the presence and absence of crosslinker appear to be the same, and the small difference in apparent Mr in the labelled components is likely due to a difference in conformational constraints arising from the two labelling processes, with a true, corrected Mr of 43,400. Since the specific labelling of this component is related to physiologically relevant binding in several bovine tissues, we conclude that it is a component of the bovine endothelin receptor.

Vascular A10 cell membranes contain an endothelin metabolizing neutral endopeptidase

We have investigated the possible presence of endothelin-metabolizing neutral endopeptidase (NEP, EC 3.4.24.11) on A10 cell membranes using [125I]-ET-1 binding and direct measurements of NEP. NEP activity of A10 cell membranes has been compared to that of solubilized rat kidney brush border membranes (KNEP). Specific [125I]-ET-1 (50 pM) binding (defined with 100 nM ET-1) to A10 cell membranes was increased in a concentration dependent manner by the selective NEP inhibitors thiorphan, phosphoramidon, and SQ 28,603 [(+/-)-N-[2-(mercaptomethyl)-1-oxo-3-phenylpropyl]-beta-alanine] with EC50 values of 9.4, 28.4, and 5.7 nM respectively. At equilibrium (150 min), 70% more specific binding was apparent in the presence of these inhibitors. Phosphoramidon (2 microM) did not alter Bmax values, but it decreased the apparent KD for [125I] ET-1 from 63 (+/- 3) to 27 (+/- 2) pM. Thiorphan, phosphoramidon, and SQ 28,603 inhibited A10 cell NEP activity with IC50 values of 5.3, 36.5, and 6.0 nM respectively, which was similar to values obtained with KNEP (3.6, 22.6, and 3.5 nM). ET-1 inhibited A10 cell NEP, and KNEP with IC50 values of 30 and 21.3 microM respectively. The order of inhibitory potencies: ET-3 greater than ET-1 = ET-2 greater than or equal to sarafotoxin-6b was similar for both systems. These data suggest A10 cell membranes contain a NEP which has similar characteristics to NEP 24.11, and which actively metabolizes [125I]-ET-1.

Are several G proteins involved in the different effects of endothelin-1 in mouse striatal astrocytes?

High-affinity specific receptors of endothelin (ET-1) were identified on primary cultures of mouse embryo striatal astrocytes by binding experiments performed with 125I-ET-1. Stimulation of production of inositol phosphates, a biphasic increase of the intracellular calcium concentration, and inhibition of cyclic AMP accumulation were observed in the same cells under ET-1 stimulation. Pretreatment of these cells with Bordetella pertussis toxin affected these effects to different extends, an observation suggesting that they are mediated by multiple transduction pathways, possibly involving several guanine nucleotide-binding proteins.

Functional endothelin-1 receptors in rat astrocytoma C6

Rat astrocytoma C6 cells have been recently identified as target cells for ET-1, which stimulates inositol lipid turnover in these cells. It is shown here that binding of ET-1 to high-affinity receptors on C6 cells leads to 40-45% inhibition of isoproterenol-induced intracellular cyclic AMP accumulation, as well as to stimulation of inositol lipid turnover, both effects characterized by an absolute requirement of extracellular calcium. Moreover, ET-1, which has been generally reported to have a mitogenic effect on a variety of target cells including primary rat astrocytes, is shown here to stimulate or, alternatively, inhibit DNA synthesis in C6 cells, depending on the subclone considered.

Identification of endothelin receptors in cultured cerebellar neurons

We have described the binding of [125I]endothelin-1 (ET-1) to cultured rat cerebellar granule cells. Binding of [125I]ET-1 was specific, saturable, and time-dependent. Scatchard analysis of saturation binding data indicated a single class of high affinity binding site with a KD of 95 pM, and a Bmax of 8110 receptors/cell. Functionally, the binding of ET-1 stimulated phosphatidylinositide (PI) hydrolysis in a dose- and time-dependent fashion. PI turnover was found to be inhibitable by 1 microM phorbol dibutyrate but not by 1 microgram/ml pertussis toxin, suggesting that the ET-1-mediated response is regulated by protein kinase C and a pertussis toxin-insensitive guanine nucleotide (GTP) binding protein.

Comparison of the bronchopulmonary and pressor activities of endothelin isoforms ET-1, ET-2, and ET-3 and characterization of their binding sites in guinea pig lung

In anesthetized and ventilated guinea pigs intravenous injection of ET-1, ET-2, or ET-3 (1 or 2 nmol/kg) induced similar dose-dependent increases in pulmonary inflation pressure (PIP) associated with increases in mean arterial blood pressure (MBP). Pretreatment of the guinea pigs with 1 mg/kg intravenous indomethacin significantly inhibited the increase in PIP evoked by 2 nmol/kg of ET-1, ET-2, or ET-3. In contrast, the increase in MBP following injection of the various ET isotypes was not significantly affected by indomethacin. Injection of ET-1, ET-2, or ET-3 (40, 120, and 400 pmol) via the pulmonary artery of isolated and perfused guinea pig lungs induced dose-dependent increases in PIP and pulmonary perfusion pressure (PPP), thromboxane B2 (TXB2) release, and formation of lung edema. In keeping with the in vivo results, no marked differences were observed between the activities of ET-1, ET-2, and ET-3 on isolated and perfused guinea pig lungs. Indomethacin (5 microM) added to the perfusion medium significantly inhibited the alterations of PIP and PPP, TXB2 release, and edema formation evoked by 400 pmol ET-1, ET-2, or ET-3. High-affinity binding sites for ET-1, ET-2, and ET-3 exhibiting similar characteristics were identified on guinea pig lung membrane. Therefore ET-1, ET-2, and ET-3 exert comparable bronchopulmonary and pressor activities in the guinea pig and probably act via interaction with the same binding site. In addition, the ET-induced increase in PIP and pulmonary vasoconstriction are primarily mediated via the production of cyclooxygenase metabolites.

Endothelin receptor heterogeneity; structure activity, autoradiographic and functional studies

Radioligand binding studies were used to define the binding constants for inhibition of [125I]endothelin-1 binding by endothelin-1 (ET-1), endothelin-3 (ET-3) and a synthetic analogue of ET-1, [Ala1,3,11,15]ET-1 in rat aorta and cerebellum. The inhibition curves for the three substances were similar in the cerebellum (Ki, ET-1 = ET-3 = [Ala1,3,11,15]ET-1) but differed in the aorta (Ki, ET-1 much less than [Ala1,3, 11,15]ET-1) with the ET-3 inhibition curves being biphasic, indicating binding to two sites. In functional studies in the aorta, in contrast to findings in the cerebellum, [Ala1,3,11,15]ET-1 was inactive and ET-3 was a partial agonist. These findings support the idea of heterogeneity of endothelin receptors. Autoradiographic studies were performed to identify the cellular localisation of endothelin-1 receptors in the two tissues. In the aorta binding was to all elements of the vasculature including the endothelium, smooth muscle, periadventitial connective tissue and nerves. In the cerebellum binding sites were located predominantly over the neuronal elements of the granular cell layer. The cellular localisation of the high and low affinity sites for ET-3, and the functional significance of the high affinity binding site for ET-3, remain to be determined.

Endothelin action in rat liver. Receptors, free Ca2+ oscillations, and activation of glycogenolysis

High affinity binding sites for endothelin (ET) were identified on rat liver plasma membranes. Binding of 125I-ET-1 with its site was specific, saturable, and time dependent (kobs = 0.019 +/- 0.001 min-1), but dissociation of receptor-bound ligand was minimal. A single class of high affinity binding sites for 125I-ET-1 was identified with an apparent Kd of 32.4 +/- 9.8 pM and a Bmax of 1084 +/- 118 fmol/mg protein. ET-3 and big-ET-1 (1-38) (human) inhibited 125I-ET-1 binding with IC50 values of 1.85 +/- 1.03 nM and 43 +/- 6 nM, respectively. Aequorin measurements of cytosolic free Ca2+ in single, isolated rat hepatocytes showed that ET-1 at subnanomolar concentrations induced a series of repetitive, sustained Ca2+ transients. ET-1 had no effect on cAMP production. Finally, ET-1 caused a rapid and sustained stimulation of glycogenolysis in rat hepatocytes. A 1.8-fold maximal increase in glycogen phosphorylase alpha was observed at 1 pM ET-1, with an EC50 of 0.03 pM. Stimulation of the enzyme was specific for ET-1 since the order of potency of related peptides was similar to that in binding experiments (ET-1 greater than ET-3 greater than big ET-1). These data constitute the first demonstration of the presence of ET-1 binding sites in liver which is associated with a rise in cytosolic free Ca2+ and a potent glycogenolytic effect. We conclude that ET-1 behaves as a typical Ca2+ mobilizing hormone in liver.

Distribution of endothelin-like immunoreactivity and mRNA in the developing and adult human lung

Localization and characterization of endothelin-producing cells in the developing (fetal and postnatal) and adult human lung was investigated using the technics of immunocytochemistry and in situ hybridization. Immunoreactivity for endothelin was seen mainly in pulmonary endocrine cells of developing human lung. Immunoreactivity was also seen in the airway epithelium in fewer cases (about 50%) of human adults. In situ hybridization with 35S- or 32P-labeled RNA probes complementary to endothelin-1, -2, and -3, showed that endothelin mRNAs were expressed in a number of cells that were in similar sites to endocrine cells. Immunocytochemistry and in situ hybridization employed on pairs of reverse-face serial sections showed the presence of endothelin immunoreactivity and mRNAs in the same endocrine cell. Correlative studies revealed that endothelin is co-localized with general endocrine markers (synaptophysin, chromogranin, protein gene product 9.5) and regulatory peptides (e.g., gastrin-releasing peptide). The density (cells/mm2) of endocrine cells containing immunoreactivity or mRNAs was highest during fetal life and started to decline before birth, and was minimal in adults. Endothelin-like immunoreactivity and mRNAs were also expressed in endothelial cells. From these results, it is concluded that endothelin is synthesized in endocrine cells of human lung and the change of developmental expression of this peptide suggests it may play a part in growth regulation in addition to its putative vasoconstrictor role in human lung.

Endothelin in the gastrointestinal tract. Presence of endothelinlike immunoreactivity, endothelin-1 messenger RNA, endothelin receptors, and pharmacological effect

The possible production and role of endothelin in the gastrointestinal tract was investigated in rats by radioimmunoassay, Northern-blot hybridization, receptor assay using membrane preparations, and pharmacological study using gut strips. Endothelinlike immunoreactivity was detected in all regions (from stomach to colon) of the rat gastrointestinal tract (13-48 fmol/g wet tissue) including the mucosal layer of the ileum and colon (8.4 +/- 2.0 fmol/g wet tissue and 18.4 +/- 2.1 fmol/g wet tissue, respectively, mean +/- SEM; n = 5). Fast protein liquid chromatographic analysis of the endothelinlike immunoreactivity in jejunum, ileum, colon, and colon mucosa extracts showed peaks in the positions of endothelin-1 and endothelin-3. The presence of endothelin-1 messenger RNA was demonstrated by Northern-blot hybridization in the whole colon and pooled ileal and colonic mucosa, but not in the whole jejunum. Specific binding in the rat gastrointestinal tract was particularly high in the fundus of stomach, jejunum, ileum, and colon. In the ileum, many binding sites were found in the circular and longitudinal muscle layers, but few in the mucosal layer. Endothelin-1 and endothelin-3 caused contraction of rat stomach strips, rat colon, and guinea pig ileum. These findings indicate that endothelin is present in the rat gastrointestinal tract, perhaps produced by both vascular endothelial cells and mucosal epithelial cells, and can cause contraction of gastrointestinal smooth muscle. Thus, endothelin may have a physiological role in the control of gastrointestinal function.

Detection of endothelin immunoreactivity and mRNA in pulmonary tumours

Paraffin sections of 66 surgically resected lung tumours were immunostained with antisera to human endothelin-1 and to the C-terminal peptide of big endothelin. With both antisera, strong immunoreactivity was demonstrated in 11 of 15 squamous cell carcinomas and 11 of 16 adenocarcinomas. Focal immunoreactivity was seen in small cell carcinoma (2/12), large cell carcinoma (2/5), and carcinoid tumours (2/11). Four lymphomas and three sarcomas did not show endothelin immunoreactivity. Cryostat sections of 22 of the 66 tumours were hybridized with radiolabelled complementary RNA probes prepared from the 3' non-coding region of endothelin-1 cDNA, and the chromosomal genes encoding endothelin-2 and -3. In situ hybridization demonstrated the presence of endothelin mRNAs in 4 of 7 squamous cell carcinomas and in 5 of 8 adenocarcinomas, in a pattern similar to that shown by immunocytochemistry. No hybridization signals were obtained from the other types of tumours. In lung tissue adjacent to the tumours, endothelin-like immunoreactivity and mRNA were detected in pulmonary endocrine cells and, in some cases, other epithelial cells, and in alveolar capillary endothelial cells. This study demonstrates the expression of endothelin in a number of pulmonary tumours and suggests a possible role for this peptide in the growth and/or differentiation of these tumours.

Uptake and metabolism of endothelin in the isolated perfused rat lung

The uptake and metabolism of endothelin was studied in the isolated perfused rat lung. 125I-Labeled endothelin (4 x 10(-12) M) was rapidly removed from the lung perfusate. More than 90% of the label was removed by 5 min, and only 4-5% remained in the perfusate after 30 min. In the presence of 10(-9) or 5 x 10(-9) M unlabeled endothelin added 6 min before 125I-labeled endothelin, the radiolabel was more slowly removed, so that after 30 min 19 or 73% of the label, respectively, remained in the perfusate. Although a high level of unlabeled endothelin in the perfusate reduced the uptake of 125I-labeled endothelin, it did not displace radiolabel previously taken up by the lung. Analysis of radiolabel composition showed that endothelin metabolism was less than 15% in both the lung and the perfusate after 30 min of lung perfusion. Autoradiography of the lung indicated that the radiolabel was located primarily within the alveolar wall. These results suggest that circulating endothelin is readily but finitely taken up in the pulmonary microvasculature where it is avidly bound but slowly metabolized.

Contractile activity of three endothelins (ET-1, ET-2 and ET-3) on the human isolated bronchus

1. The effects of three endothelins: (i) the classical or human/porcine endothelin (ET-1); (ii) [Trp6, Leu7] endothelin (ET-2) and (iii) [Thr2, Phe4, Thr5, Tyr6, Lys7, Tyr14] endothelin or rat endothelin (ET-3) were tested on the human isolated bronchus. 2. ET-1 produced a concentration-dependent contraction of the human isolated bronchus that proceeded in two different steps. The first step was observed at very low concentrations (pD2 = 11.01 +/- 0.17, n = 10) but corresponded to a low intrinsic activity (Emax = 15.6 +/- 1.8% of Emax = 26.1 +/- 2.9% of ACh 3 x 10(-3) M, n = 5, P less than 0.05), reduced by nicardipine 10(-6) M (Emax = 6.0 +/- 2.6% of ACh 3 x 10(-3) M, n = 5, P less than 0.05) and strongly inhibited in calcium-free medium. The second step of the action of ET-1 corresponded to a lesser potency (pD2 = 7.90 +/- 0.17, n = 9) but a higher intrinsic activity (Emax = 82.5 +/- 4.7% of ACh 3 x 10(-3) M). This effect was not significantly modified by nicardipine 10(-6) M or by Bay K 8644 10(-7) M. Neither of the two effects was modified by indomethacin 3 x 10(-6) M. 3. The effects of ET-2 and ET-3 were qualitatively similar to those of ET-1 but quantitatively different; for these two steps of contracting activity and for potency and efficacy the ranking was: ET-1 greater than ET-2 = ET-3. 4. Thus, ET-1 appears to be the most potent of these three substances in its effect on the human isolated bronchus. Its activity seems to involve the action of voltage-dependent calcium channels at low concentrations (10-12 to 10-9M), whereas other mechanisms are involved at higher concentrations (10-8 to 3 x 10-7M).

Elevated plasma endothelin in patients with diabetes mellitus

Plasma concentrations of endothelin, a vasoconstrictor peptide released from vascular endothelial cells, have been measured by radioimmunoassay in 100 patients with diabetes mellitus and 19 healthy subjects. The plasma immunoreactive-endothelin concentrations were found to be greatly raised in the patients with diabetes (1,880 +/- 120 fmol/l, mean +/- SEM) compared with the healthy subjects (540 +/- 50 fmol/l, p less than 0.005). The elevation of immunoreactive-endothelin could not be explained by secondary changes in blood pressure or renal disease and did not correlate with the presence of diabetic retinopathy, duration of diabetes mellitus, fasting blood glucose or serum fructosamine. Fast protein liquid chromatographic analysis of the diabetic plasma immunoreactive-endothelin showed three forms, one in a very big molecular weight position, one intermediate and one in the position of endothelin-1 itself. No material appeared in the positions of endothelin-2 and 3. Chromatographic analysis of normal plasma showed only the big molecular weight peak while material in the endothelin-1, 2 or 3 positions was below detection. The elevation of endothelin in diabetic patients may be a marker of, and further exacerbate, their vascular disease.

In vitro vascular reactivity to endothelin: a comparison between young and old normotensive and hypertensive rats

In this study we have investigated whether the vascular smooth muscle of a large capacitance artery of spontaneously hypertensive rats (SHR) is hyperresponsive to endothelin-1 and whether the arterial responsiveness to endothelin-1 is affected by aging. Isometric contractions of spirally cut aortic strips from SHR of 11, 22, 33 and 44 weeks of age and from age-matched WKY were measured in parallel. The vessels from SHR did not exhibit a greater responsiveness to endothelin-1 than those from WKY. No difference of responsiveness to the peptide was found among the arteries isolated from WKY of different ages. In contrast, a progressive decrease of responsiveness to endothelin-1 with aging was observed in SHR. This finding seems to be specific for endothelin-1, since the responsiveness to norepinephrine was unchanged. The significant decrease of aortic responsiveness in SHR with aging might be due to chronic hypertension and indicate desensitization to endothelin-1. The latter might be related to chronic in vivo hyperproduction of endothelin, either genetically determined or related to the hypertension-induced endothelial damage.